System and method of compensating for film characteristics in photographic sound recording



June 6, 1950 R. L.. MALCOLM 2,510,592

SYSTEM AND METHOD DE coNPENsATING RoR RIEN CHARACTERISTICS IN PHoToGRAPHIc souND RECORDING Filed Nov. 1a, 194e IN VEN TOR.

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Patented `une 6, 195() SYSTEM AND METHOD OF COMPENSATING FOR FILM CHARACTERISTICS IN PHOTO- GRAPHIC SOUND RECORDING Robert L. Malcom, Indianapolis, Ind., assignor to Radio Corporation of America, a corporation of Delaware Application November 18, 1946, Serial No. 710,431

13 Claims. 1

This invention relates to photographic sound recording systems, and particularly to a feedback system which compensates for the nonlinearity of the exposure-transmission characteristic of a photographic emulsion.

Feedback systems for the elimination of distortion due to non-linearity of a modulating element, such as a galvanometer, are well-known, as evidenced by Wolfe U. S. Patent No. 2,270,367 of January 20, 1942, Singer U. S. Patent No. 2,292,166 of August 4, 1942, and Albin U. S. Patent No. 2,357,623 of September 5, 1944. Another patent pertinent to the present invention is Blaney U. S. Patent No. 2,361,451 of October 31, 1944, which discloses and claims a method of and system for controlling the exposure of a film in accordance with the emulsion characteristie of the film.

The present invention is directed to a system of compensating for the non-linearity of the lm emulsion by the use of a control film having a density or transmission variation in accordance with a control lm actually made on the recording system in which it is to be subsequently used. In this manner, a particularly accurate compensation can be achieved, and the linear recording range extended between more widely separated limits.

It is well-known that the negative exposurepositive transmission characteristic of a photographic emulsion has a central linear portion and curved end portions. As long as the exposure varies between the limits of the linear portion of a characteristic, true fidelity is obtained; but, when the exposure exceeds these limits, distortion is introduced, since the transmission does not vary linearly in accordance with the exposure. By varying the exposure over the curved portion of the characteristic to compensate for the departurefrom linearity in accordance with the amplitude of the signal, the final sound record willfaithfully correspond to the original signal.

The principal object of the invention, therefore, is to facilitate the recording of photographic sound records.

Another object of the invention is to provide an improved method of and means for recording photographic sound records.

A further object of the invention is to provide an improved method of and means for producing a sound record having linearity over a wider range than normal.

A still further object of the invention is to provide a feedback system for a photographic recording system in which a control lm is used to provide compensation for the non-linearity ol' the film emulsion.

A still further object of the invention is to provide a feedback system for a photographic recording system in which a compensatory conn trol lm is produced by the recording system for use in the feedback system.

Although the novel features which are believed to be characteristic of this invention will be pointed out with particularity in the appended claims, the manner of its organization and the mode of its operation will be better understood by referring to the following description read in conjunction with the accompanying drawings, forming a part hereof, in which:

Fig. 1 is a graph of a composite characteristic between positive transmission and negative eX- posure, mirror displacement, and control film density.

Fig. 2 is a diagrammatic view of a photographic recording and feedback system embodying the invention, and

Fig. 3, Fig. 4, and Fig. 5 are diagrams illustrating the relative positions of the light beam with respect to the recording slit mask and control beam mirror.

Referring now to Fig. 1, the solid line curve represents an exposure-transmission characteristie having a linear portion between points A and B and toe and shoulder curved portions. It is well-known that as long as the exposure is between the points A and B, the light trans-` mitted through the lm will be directly propor-y tional to the exposure; but, when the exposure is less than at point A or greater than at point B, there Will be a non-linear proportionality, as illustrated. The present invention is adapted to approach very closely the condition wherein the straight portion A-B is extended, as shown by the dotted lines, to permit undistorted operation over the range of negative exposures between C and D. Although the portion A-B may not be absolutely rectilinear, it may be considered as such for purposes of explanation, the invention, however, correcting for any departure from rectilinearity regardless of its position on the characteristic. ,Y Y

The same curve may be translated into terms of positive4 transmission and galvanometer mirror displacement when such a modulating element is used, 91 Arepresenting the maximum angular displacement of the mirror in the direction indicated by the positive values of the signal current, and oz, the maximum displacement in the negative direction. The curve also shows the variation of the transmission through the control nlm made in accordance with the invention with displacements of the galvanometer mirror as will be explained hereinafter.

Referring now to Fig. 2, I is a .lightsource having a straight filament, the :light therefrom being collected by a lens 2 and projected onto a galvanometer mirror by a lens 5. Intermediate the lenses 2 and 5 is a mask 4 having an aperture I6 therein and an edged-1 for'forrning a penumbra of uniform intensity :.gradient. A lens 3 is so positioned as to aid in getting a sharp image of slit I6, but does not appear in the line of light contributingto 'the penumbra produced by the edge I1.

Light, passing the edge I1and fromthe mirror 6 which is vibrated by a galvanometer', is reiiected to a lens 1 in front of a slit maskiSLI having a slit 8 therein. Light emerging through the slit .8 Vis projected by .a .lens A9 -onto the isound track `portion of a 4nlm Il). By .the dotted lines of the mirror 6, the limiting excursions .of the mirror E are represented between .positions 91 and e2, `as .shown in Fig. 1.

Light through the aperture I6 is reected by the mirror to a stationary .mirror II where it is reiiected through a lens I2 'to -a curved surface i3, which, during recording, vis a control lm. Shown at I8 and I9,.are two mirrors, which, `when rotated to the positions shown-by .the dotted-lines, reflect the penumbra shadow back through aperture I5 which is then reflected by the mirror 6to the mirror II and through .lens I2tovcurved surface I3, the points 91 rand y92 representing `the maximum excursion limits .of the mirror, as shown in Fig. 1. When the .mirrors .are in their dotted line positions, thecontrol lmfor use .during recording is exposed in .its preparation.

To produce the control strip I3, a striplofnnexposed negative stock is exposed so that .from position 91 to 92 there is a uniform lexposure gradient. This strip is then developedand .a,posi tive strip printed from it. `The positivestrip is then placed accurately along the .surface I3 --so as to maintain the original register of exposure. To expose the original negative, lthe .mirror 6 is uniformly moved between its -twoiextreme positions 61 and 62. The mirrors -I8 and-ISiare-then returned to their normal operating positions, shown by the solid lines. The positive strip 'developed from the negative 'is then place'dat I3, the slit i6 being sharply focused thereon. The recording system is now ready for use in conjunction with the remainder of the system which will now be described.

Light, emerging through the positive strip I3, is projected by lens Ill onto the photoelectriccell I5 connected to an amplifier 2D havinglow phase distortion. The input signal to the galvanometer 3d arrives'at terminals 24 from any suitable signal generating device,.such as amicrophone ora sound reproducer, the signalbeingimpressedupon an amplifier 26. Amplifier 26 has .one output over conductors 32, 33, and 3.4 to amplifier 2l which is connected over conductors `35 tothe galvanometer 3l), anda second output over v'con-- ductors 3? and 38 to amplifier 21, conductors `31 and 33 having a, resistor Yor :potentiometer 29 therein for controlling the amplitudeoffthe signal impressed upon amplifier 2.1. That is, .the attenuator 29 is so adjusted thattheoutput of yampliner 21 balances, or isfequal to, theoutput of amplifier 20 when lthe image fof slitfIB -isinfocus on the linearportion of the control strip I3. `Inserted between the output of amplifier 21 and the output of ampliiier 20 is an impedance 28 which is adjustable for varying the voltage impressed upon amplier 2| in addition to that from amplier 26. The output current from amplifier 21 is of such polarity or phase that it augments the voutputcurrent from amplifier 26, while the output currents from amplifier 2i! are phased to oppose the output currents from ampliner 26.

In normal operation of the system just described, vthe penumbra is vibrated across the slit 8 `bytlu-ymirror '6 in accordance with the amplitude and frequency of a signal impressed upon thegalvanometer 1.33, while the light through the aperture I6 is'impressed upon the control nlm I3 for impression-on the photoelectric cell I5 in .accordance `with the'amplitude and frequency of the signal. When, at certain input signal levels, galvanometer mirror Sproduces an exposure on negative iilm Ill within the linear range, the image of slit :IB- is Valso at a corresponding. position of -linearity.',a'longcontrol;stripf',l 3. -.Over this grange, vthe J.output vfrom gamplier A2li is balanced 4yby lthe output from-'amplifier 1, and; no control potential is developedzacrossximpedancesZ. This4v range-.is between .A and vZB .1 on thecurve finfFig. s1.

At'another value Vof ,inputrsignalsand .corr-esponding angular displacementfof .the 4galvancmeter mirror, the :resulting-'effectiveexposure ,at .negative I0 maybe such;that it fails tofreach the ,condition of linear ,opacity afterfdevelopment'of the negative. Thus,v.while perfectlydinearemulsion .system would ;give.;linear positivetransmissionfor linear exposure, the emulsionbeingfconsidered .will .b e assumed ...to give .lessthan the requisitetransmission through its positive. fAt .thisvpositionof y'mirror-n the imagefoslit IB isat a position of corresponding non-,linearity-ialong .the .surfaceof control strip y[3,resultinginan-:outputv current from amplifier 20 i-that fis -gless than thatifromfamplifler -2.1. Due to thisf laclcof balance, -there .is developed across .impedance-28, .a current that .augments :the output .from ampliner-26. Theputput fromfamplier 2| being-.thus increased, the galvanometer tends Vtin-.assume ,a position providing =more illuminationifor the `exposure of.ne gativer.l0. Whenmirror has -advanced to a position whereby the fimvagerof slit IB has .reached such Ta point alonggcontrolstrip I 3 Vthat' balance .tends -.to .be restoredindmpedance 2-8, the feedback .potential tendsytoease increasmg.

.Whenthe `exposure .oflm .lfoccursinfaregion where the positiver transmission is greater ...than that .indicated .by linear conditions, the .output of ,amplifier ZILeXceeds lthat of amplier ,and the lunbalance potential .developed :across impedance 28 v.tends to oppose kthe 'input Vto 4amplifier 2|, vthereby resultingr in a tendencyfor'the .illumination ofexposure `toV-bereduced. A

By proper selection-.ofthe :.amountrof-.feedback current derivedfromfimpedance .28, kit `is Ipossible to approach, at will, the condition ofloverallnegative exposure-positive transmissioncharacteristic --with ,any type Aof .emulsion ordinarily used. This is because the controlled strip `I-.S-.may-.be made .with .the recording :system .justcdescribed and `thereafter usedtherein. `By .making .the dimensions of the control lm I3 Yandthe imageof slit i5 very large with respect-.tothe average,- grain size,.ground noise -will-be minimized.

.In Figs. 3, `and 5, three positions :of Y thge penumbra shadow, shown graduated :fromflight at the atop .to fdark .at .the r.lcxottorrn .aref'shown impressed .inthree positions `on-the.-slit 8 and .the

fiirror Il together with an image 25 of the aperture I6 thereon. As the mirror 6 vibrates throughout its excursion, the penumbra will travel across the slitr 8, While the image of aperture I6 will move across the mirror Il. Therefore, regardless of Whether a high or low intensity light is being passed by the slit 8, a constant intensity light image 25 is being projected on the control lm strip I3 which is modified in intensity by the density of the film control strip, as shown in Fig. 1, which corresponds to the emulsion characteristic of the original film.

The system may also be used in push-pull class B variable density systems by using two directionally opposed and laterally displaced penumbra and light slit channels. Properly adjusted, this system and method of feedback also tends to reduce distortion eects introduced at any point in the feedback loop, such as amplier 2i and galvanometer 3), the latter including reflected mechanical properties. To provide the least possible delay distortion at all frequencies in the range being recorded, high frequency carrier currents may be used and all gain be effected by wide band amplification at these high frequencies, with power detectors forming the output circuits to the various loads.

Should it be economically desirable to accomplish vdynamic control of film exposure with less equipment than that already described, the electrical portions of this system may be simplified considerably and satisfactory operational stability may be retained by applying the amplified current from the photoelectric cell as a degeneration factor directly into the amplifier channel that drives the galvanometer. This eliminates the balancing arrangement. In this event, it is only necessary to consider the exposure characteristics of the film emulsions used, together with the conditions of feedback, and to compute the variable manner in which the progressive exposure characteristic shall proceed along the length of the control strip negative. Alternatively, the control strip may be computed and laid out as a variable area mask so that various lengths of the image of the control light slit -are presented to the condenser lensphotoelectric cell combination. When this is done, there is the advantage that this element of the system, namely, the control strip, introduces no background noise.

It is also possible to obtain the compensation current from a separate system using a galvanometer as nearly identical as possible to the one in the actual recording channel, thus providing conversion apparatus for existing systems. This auxiliary galvanometer would, of course, be actuated by amplified signal current obtained ahead of that point in the circuit at Which degeneration is introduced.

I claim:

l. A sound recording system comprising a source of light, means for forming said light into a beam having a uniformly varying intensity gradient, a slit mask having a slit therein, means for vibrating said light beam across said slit, a lm having an emulsion thereon, said emulsion having a non-linear exposure-transmission characteristic, a second film having a density variation varying in accordance with the exposuretransmission characteristic of said emulsion, means for forming said light into a second beam having a uniform intensity throughout its crosssectional area, said vibrating means vibrating said second beam over said second lm, and a feedback circuit adapted to vary said vibrating means in accordance With the variation in density of said second film to compensate for the nonlinear characteristic of said emulsion, said circuit including a detector of said second beamand aconnection to said vibrating means.

2. A sound recording system comprising a source of light, means for forming said light into two beams, one of said beams having a uniformly varying intensity gradient, and the other of said beams being of constant intensity, a film emulsion having a non-linear exposure-transmission characteristic, means having a variation in density corresponding to said exposure-transmission characteristicr and means for simultaneously vibrating both of said beams, said varying intensity beam being impressed upon said film emulsion in accordance with the amplitude and frequency of a signal being recorded, and said beam of constant intensity being vibrated over said means having a density varying in accordance with the exposure-transmission characteristic of said emulsion` 3. A sound recording system in accordance with claim 2, in which a detecting means for light passing through said means having a density varying in accordance with the exposuretransmission characteristic of said emulsion is Y provided together with a feedback system between said detecting means and said vibrating means for varying the portion of said varyingfintensity beam impressed on said emulsion in accordance with'the variations in density of said varying density means.

4. A sound recording system in accordance with claim 2, in which a feedback circuit is provided between said constant intensity beam and said vibrating means for varying the actuation of said vibrating means in accordance with the position of said constant intensity light beam on said variable density means, said circuit including a light-to-current translator for detecting variations in intensity of said beam.

5. The method of sound recording comprising generating a light beam of uniformly varying inq tensity gradient and a light beam of constant cross-section and constant intensity, simultaneously vibrating said beams, impressing said beam of varying intensity on a light sensitive emulsion in accordance with the amplitude and frequency of a signal being recorded, and varying the impression of said beam on said emulsion in accordance with the position of said constant intensity beam.

6. The method of compensating for the nonlinearity of the exposure-transmission characteristic of a film emulsion during recording comprising producing two light beams, one of said beams having a uniformly varying intensity gradient, and the other of said beams having a constant intensity, impressing portions of said beam of varying intensity on a film emulsion in accordance with the amplitude and frequency of a signal being recorded, and simultaneously Varying the position of said constant intensity beam over a surface varying in light transmission in accordance with the variation in the exposure-transmission characteristic of said emulsion.

7. The method in accordance with claim 6 which includes generating a current from said constant intensity beam as modified by transmission through said surface for varying the position of said varying intensity beam.

8. The method in accordance with claim 6 which includes generating a current from said constant intensity beam as modied by transfais-rosea mission through :said smfacefand "utilizing .said currentiformodifyingllthe impression of: said y.var-ying intensitybeamronasaidemulsion.

9.l'12he method lof compensating -for the nonh linearity of the exposure-transmission character- :istic fof a vlmvnemnlsion r'during recording comuprising generating two ilghtfbeams, one of said beams vhaving :a uniformly varying .intensity gradient, "and Athe :Sather-Eef :said beams bei-ng of 'constant size and .intensity, projecting 4portions @of-said var-ying intensity-beam to a lm 'emulsion `having -amori-linear exposureltransmissionchariacteristic, :the .portion fof :said lb'eam being-projected :at any Ainstant Vdepending upon the amplivtud'e :oflthesig'nal being recorded, 'varying the i rinten-sity of. saidv 'constant intensity 'beamfin accordance 'with Lthepor-tionloff'the varyingin'tensity lbeam Seeing-impressed Lupen said emulsion, and :controlling the-'portion Ao'f saidvvar-iable 'intensity :beam impressed :on said emulsion by .the -ariartions 2in sadfconstantfintens'ity beam.

10. The method in accordance with c1aim'9, in Wlhichfsaid last ymentioned `controlling supple- 'ments the 'variation ,of 4said variable intensity 'beamby the anip1itude of said signal.

A11..Asound recording vsystemYcomprising-a light source, .means 'for 'fforming :a plurality of light beam'sffrom said source, one'of saidbeamsfhavin'g Aa .uniformly'varyngsiintensity gradient, 'and the -other .ofsaid kbeams'lnaving a, constant `size and intensity, Vmeans ifor :simultaneously vibrating said beams, ;a rst flm,zsai'd Vvibrating lmeans being adapted to impress different portions iofsaid vary-ing intensity beam .on said lm, :a second nlm-means funpmjectingisaidaanstaat: intensity 'beamtosaidiseeondlm;fandJ meansfor1 lir'riinatiing p:said varying-:intensity beam from said -rst film and pro'j acting it-lto# said Vsecond-film 'iinst'ead 'of saiidfcens'tant iintensitybearn.

`12. Y l-A system accordance with lclaim K11, Aiin which said hiibrating means fa galvanometer -mirror and said'ilastmention'e `meansfcomprises lgl'htkieetors, one of which ishpostione'd vin the 'path-:of fsaid 'variable Yintensity `beam Aafter tie- 'e'ctie-nfby said-mirror.

13. A system in accordance:Withfclaim T11, iin whichis'aid-ilast lmentionedmeans includes alight fdenector de'fiecting .said :varying :intensity beam ifromfzsa'id Vibrating means Landia Sdeflectorrinter- 'mediate 'said liglitfs'ource and saidbeam forming imeans for :zfnrther -iienecting said varying fintensity'beain.

RGBERT L. MIA-BCCM.

REFERENCES CITED "Thefllowing Ireferences are of record lin 'the le of this patent':

UNITED 'STTES 'PATENTS .Number yName Date 2,105,769 Hansen Jan. 118, '1938 2,1198398 :'Albershein Apr. .30, V-1940 2,268,097 Underhill Dec. 130, 11941 2,286,729 Hal-1 June @16,1942 `2,292,166 Singer f r Aug. 4, 1942 2,351,623 Albi-n v Sept, 111944 12,361,451 Blane'y Oct. 311, 1944 2,376,567 .Albin May 22,1-945 

